I suppose

 > (setq pop-up-windows                nil
 >       pop-up-frames                 t
 >       special-display-buffer-names  '(("*Calendar*" (same-frame . t)))
 >       display-buffer-mark-dedicated 'weak)
 >
 > Under this configuration, pop-up-frames are used, with windows dedicated
 > to their buffers, but the calendar is opened in the current frame, not a
 > new frame.  Calendar-hide-window seems to assume that whenever dedicated
 > windows are being used, it is okay to delete or iconify a frame:
 >
 >   (cond
 >    ...
 >    ((and (display-multi-frame-p) (window-dedicated-p window))
 >     (if calendar-remove-frame-by-deleting
 >         (delete-frame (window-frame window))
 >         (iconify-frame (window-frame window))))
 >    ...
 >    )
 >
 > M-x version:
 >
 >   GNU Emacs 23.4.1 (x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 >   of 2012-04-07 on trouble, modified by Debian
 >
 > Any thoughts for a solution?

The corresponding code has changed on trunk but the bug is still
present.  Note that you now have to

(setq calendar-setup t)

in order to reproduce it.  I'd propose the attached patch which still
has the deficiency that KILL is not respected when the buffer appears on
a separate frame.  This is, however, a general problem shared, for
example, by `debug' which kills the *Backtrace* buffer when it appears
on the same frame and retains it when on another.

martin